11 2 The Hybrid Ecosystem A Hypothesis Urban ecology faces a significant challenge: to position itself in the context of planetary change and to understand the role that cities play in the evolution of Earth. The dynamics of urban ecosystems are complex and hard to predict because we do not yet understand how they work. Here I advance the hypothesis that cities are hybrid ecosystems, the products of coevolving human and natural systems. Building on ecology and evolutionary theory, I contend that it is cities’ hybrid nature that makes them unstable and unpredictable, and simultaneously capable of innovation. This chapter examines complexity, emergence, resilience, and innovation in urban ecosystems and the scientific challenges that they pose to urban ecology. A science of cities as novel ecosystems has yet to be developed. Cities are the most visible signature of the Anthropocene: a new era in the co-evolution of life and the planet (Crutzen and Stoermer 2000; Monastersky 2015). Yet our understanding of how cities, as coupled human-natural systems, emerge, grow, and evolve is at best tentative and fragmentary. Despite remarkable progress in the study of urban ecosystems over the past few decades, urban ecology still lacks a theory of human habitat that is comparable to ecological theories of natural habitat. What makes the city an optimal habitat for people and their communities? Do thriving cities share certain properties? Can we identify a set of features that explains these cities’ vitality and well-being? Are there factors, such as size, density, or form, that best correlate with the health of cities? What makes cities able to adapt, change, and evolve through time? What makes a city resilient? 12 Chapter 2 The study of cities has evolved through many theories, myths, and paradoxes. Cities have been compared to living systems (Geddes 1915), biological organisms (Odum 1971), and ecosystems (Odum 1975). Kevin Lynch (1981) was among the first to point out that theories of urban genesis reflect dominant images of the evolving conception of the relationship between humans and nature. Cities mimic biological systems in many ways, yet they exhibit characteristics that break many known natural rules. More than half a century ago, Jane Jacobs (1961) referred to the city as “organized complexity.” Since then, the view that cities are complex systems has emerged as a new challenge for science (Batty 2005). Like living systems, cities are diverse and complex and yet governed by extraordinarily simple universal laws (Bettencourt et al. 2007). More recently, Bettencourt (2013) compared cities to stars: as the spatial and temporal scale of cities increases, social interactions within them intensify. This process is analogous to nuclear reactions occurring within stars: the larger the population of a city, the higher the rate of its social interactions. Like large stars that burn faster and brighter than smaller ones, cities with larger populations attract even more people, and their social interactions accelerate (ibid.). Yet scientists recently have found that the structure of human interactions in cities has some unexpected underlying qualities. On average, as the size of a city doubles, the total number of social interactions within it more than doubles—and in a predictable way—but, regardless of a city’s size, the group clustering of its social networks does not change. Whether they live in large cities or small villages, people form tight social communities (Schläpfer et al. 2014). The main difference is that in small towns, interpersonal connections might be defined by spatial proximity, while in large cities, people form social communities through a diversity of communication networks based on affinities and/or interests. Scientists studying cities as complex systems are curious about the universal principles that govern them and which could lead to a predictive theory of urban growth and change to inform decisions about their management. A team led by Luis Bettencourt and Geoffrey West at the Santa Fe Institute (SFI) has been working toward formalizing a new quantitative understanding of urban function (Bettencourt and West 2010). By examining a large set of data on diverse aspects of urban regions, they observed that cities exhibit scaling relationships similar to The Hybrid Ecosystem 13 those that biologists have found for organisms’ molecular, physiological, ecological, and life-history attributes (Bettencourt et al. 2007). And emerging evidence suggests that such phenomena might not be limited to modern settlements. By examining archaeological settlement data from the pre-Hispanic Basin of Mexico, Ortman et al. (2015) found that scaling relationships between socioeconomic production and settlement size have characterized human settlements...

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